Vinyl acetate is an important industrial material used in a wide range of fields including coatings, adhesives, fiber treatment agents and the like, as a starting material for vinyl acetate resins, as a starting material for polyvinyl alcohols, and as a monomer for copolymerization with ethylene, styrene, acrylate, methacrylate and other monomers.
Catalysts such as Pd/Au/KOAc/SiO2 are commonly used for production of alkenyl acetates obtained using acetic acid, lower olefins and oxygen as starting materials, and especially vinyl acetate. The active site for the reaction is believed to be palladium, with the gold cocatalyst inhibiting sintering of the palladium, reducing carbon dioxide gas generation and improving the alkenyl acetate (for example, vinyl acetate) selectivity. It has been reported that, in order for gold to exhibit its effect, it has to mix with palladium on the atomic level.
An extremely important technical problem in the production of vinyl acetate is achieving increased vinyl acetate selectivity, while inhibition of carbon dioxide gas generation is also important from the viewpoint of the environmental load. Lengthening the life of the catalyst is another major issue for industrial production of vinyl acetate from an economical standpoint, and improving the function of gold is considered important for inhibiting sintering of palladium.
Catalysts for production of vinyl acetate which are of the “shell” type, having palladium or gold supported only on the surface of the carrier, are considered to have superior reactivity. Methods for preparation of shell-type catalysts are disclosed in Japanese Patent Public Inspection No. 2004-526553, for example. In this publication, the carrier is impregnated with a solution of the starting metal salts and then contacted with an alkali solution as a fixing agent to form a shell-type catalyst. However, this method forms a catalyst in which the positions of palladium and gold loaded in the shell are relatively separated, while the gold loading ratio is also unsatisfactorily low.
In addition, British Patent No. 1283737 and Japanese Unexamined Patent Publication No. 8-318159 disclose impregnation of carriers with alkali solutions first, followed by contact with starting metal salts solutions to form shell-type catalysts. For instance, the examples of British Patent No. 1283737 describe a step in which the carrier is impregnated with an alkali solution and the solution is then removed from the carrier by heating. Japanese Unexamined Patent Publication No. 8-318159, moreover, teaches that the total amount of the alkali solution and starting metal salts solution must be equivalent to the amount of water absorption by the carrier used. Yet the steps of these processes are complex and the preparation procedures difficult, while irregularities in loading of the palladium and gold tend to result.
Furthermore, Japanese Unexamined Patent Publication No. 10-175917 discloses treatment with a centrifugal separator after impregnation of a carrier with a starting metal salts solution for loading of the metal in a shell fashion, but the process is complex and in need of improvement.